PROJECT SUMMARY The Sts phosphatases negatively regulate signaling pathways within cells of the mammalian immune system. Mice lacking Sts expression (Sts-/-) are profoundly resistant to infection by clinically relevant fungal and bacterial pathogens, including Candida albicans, Francisella tularensis, and Staphylococcus aureus. Resistance is associated with rapid pathogen clearance and reduced inflammation. Our preliminary data demonstrates that phagocytes lacking Sts expression have enhanced microbicidal functions. We have shown that the Sts phosphatase domain is structurally and mechanistically distinct from other classes of protein phosphatases. We also demonstrate that the Sts active site sits in a distinct binding pocket, and enzyme activity can be competitively inhibited with drug-like compounds. We hypothesize that drug-mediated inhibition of Sts-1 will recapitulate the Sts-/- phenotype and lead to beneficial clinical outcomes. Based on this hypothesis, the objective of this proposal is to identify and characterize small molecule inhibitors of Sts-1 that can enhance leukocyte anti-microbial responses and demonstrate efficacy in whole animal infection models. To achieve this objective, we will conduct a high-throughput screen (HTS) of the 530K compound Scripps Drug Discovery Library. Active compounds will be extensively characterized and thoroughly validated, using our established assays and in vivo models. We will accomplish our objective by completing the following Specific Aims: Specific Aim 1: Conduct a 530K compound HTS to identify inhibitors of Sts-1 phosphatase activity. Specific Aim 2: Validate and characterize hit compounds. Specific Aim 3: Determine the effects of Sts inhibitory compounds on host responses to microbial infection. Successful completion of the proposed studies will yield a set of validated small molecule inhibitors of Sts phosphatase activity that will serve both as chemical probes of function and as leads for the development of novel therapeutic agents. In the long term, this work is expected to provide a foundation for the development of new clinical protocols that will significantly reduce the morbidity and mortality attributed to systemic pathogen infections, by enhancing host immune responses.